Method for re-wetting surface post-cross-linked, water-absorbent polymer particles

ABSTRACT

A process for producing water-absorbing polymer particles, wherein surface postcrosslinked water-absorbing polymer particles are remoisturized and classified, and wherein the time between remoisturization and classification is at least 15 minutes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the U.S. national phase of International Application No.PCT/EP2010/064927, filed on Oct. 6, 2010, which claims the benefit ofU.S. provisional Application No. 61/250,034, filed Oct. 9, 2009,incorporated herein by reference in its entirety.

The present invention relates to a process for producing water-absorbingpolymer particles, wherein surface postcrosslinked water-absorbingpolymer particles are remoisturized and classified, and wherein the timebetween remoisturization and classification is at least 15 minutes.

Water-absorbing polymer particles are used to produce diapers, tampons,sanitary napkins and other hygiene articles, but also as water-retainingagents in market gardening. The water-absorbing polymer particles arealso referred to as superabsorbents.

The production of water-absorbing polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, pages 71 to 103.

The properties of the water-absorbing polymer particles can be adjusted,for example, via the amount of crosslinker used. With increasing amountof crosslinker, the centrifuge retention capacity (CRC) falls and theabsorption under a pressure of 21.0 g/cm² (AUL0.3 psi) passes through amaximum.

To improve the application properties, for example permeability of theswollen gel bed (SFC) in the diaper and absorption under a pressure of49.2 g/cm² (AUL0.7 psi), water-absorbing polymer particles are generallysurface postcrosslinked. This increases the degree of crosslinking ofthe particle surface, which allows the absorption under a pressure of49.2 g/cm² (AUL0.7 psi) and the centrifuge retention capacity (CRC) tobe at least partly decoupled. This surface postcrosslinking can beperformed in the aqueous gel phase. Preferably, however, dried, groundand screened-off polymer particles (base polymer) are surface coatedwith a surface postcrosslinker and thermally surface postcrosslinked.Crosslinkers suitable for this purpose are compounds which can formcovalent bonds with at least two carboxylate groups of thewater-absorbing polymer particles.

After the thermal surface postcrosslinking, the water-absorbing polymerparticles often have a moisture content of less than 1% by weight. Thisincreases the tendency of the polymer particles to static charging. Thestatic charging of the polymer particles influences the dosage accuracy,for example in diaper production. This problem is typically solved byestablishing a defined moisture content by adding water or aqueoussolutions (remoisturizing).

Processes for remoisturizing are disclosed, for example, in WO 98/49221A1 and EP 0 780 424 A1.

Water-absorbing polymer particles can be transported by means ofpneumatic delivery systems.

Processes for pneumatic delivery are described, for example, in WO2007/104657 A2, WO 2007/104673 A2 and WO 2007/104676 A1.

It was an object of the present invention to provide an improved processfor producing water-absorbing polymer particles, especially to preventand/or improve the removal of excessively small polymer particles.

The object was achieved by a process for producing water-absorbingpolymer particles by polymerizing a monomer solution or suspensioncomprising

-   a) at least one ethylenically unsaturated monomer which bears acid    groups and may be at least partly neutralized,-   b) at least one crosslinker,-   c) at least one initiator,-   d) optionally one or more ethylenically unsaturated monomers    copolymerizable with the monomers mentioned under a) and-   e) optionally one or more water-soluble polymers,    comprising drying, grinding, classifying and surface    postcrosslinking, in which    i) the surface postcrosslinked polymer particles are remoisturized,    ii) optionally, the remoisturized polymer particles are    pneumatically delivered, and    iii) the remoisturized polymer particles are classified,    wherein the time between the remoisturization i) and the    classification iii) is at least 15 minutes.

The remoisturization i) is typically performed by adding an aqueousliquid in suitable mixing units. Suitable aqueous liquids are water,aqueous solutions and aqueous dispersions. It is advantageous to usemixers with high-speed mixing tools, since this minimizes the tendencyof the water-absorbing polymer particles to form lumps. Furtherparameters which influence the tendency to form lumps are thetemperature of the water-absorbing polymer particles and the ionicstrength of the aqueous solution used for remoisturization. The tendencyto form lumps decreases with rising temperature and rising ionicstrength.

The temperature of the water-absorbing polymer particles supplied to theremoisturization i) is therefore preferably from 40 to 80° C., morepreferably from 45 to 75° C., most preferably from 50 to 70° C.

The mixers usable for remoisturization i) are not subject to anyrestriction. Preference is given to using mixers with rotating mixingtools. According to the position of the axis of rotation with respect tothe product flow direction, the mixers with rotating mixing tools aredivided into vertical mixers and horizontal mixers. It is advantageousto use horizontal mixers for remoisturization.

Suitable horizontal mixers with moving mixing tools are, for example,screw mixers, disk mixers, paddle mixers, helical ribbon mixers andcontinuous flow mixers. The aqueous liquid can be sprayed on either inhigh-speed mixers or in mixers with low stirrer speed. A preferredhorizontal mixer is the continuous flow mixer. Suitable continuous flowmixers are obtainable, for example, from Gebrüder Ruberg GmbH & Co KG,Nieheim, Germany.

The inner wall of the horizontal mixer has, with respect to water, acontact angle of preferably less than 80°, more preferably of less than65°, most preferably of less than 50°. The contact angle is a measure ofthe wetting behavior and is measured to DIN 53900.

It is advantageous to use horizontal mixers whose inner wall which is incontact with the product is made of a stainless steel. Stainless steelstypically have a chromium content of 10.5 to 13% by weight of chromium.The high chromium content leads to a protective passivation composed ofchromium dioxide on the steel surface. Further alloy constituentsincrease the corrosion resistance and improve the mechanical properties.

Particularly suitable steels are austenitic steels with, for example, atleast 0.08% by weight of carbon. Advantageously, the austenitic steelscomprise, as well as iron, carbon, chromium, nickel and optionallymolybdenum, further alloy constituents, preferably niobium or titanium.

The preferred stainless steels are steels with materials number 1.45xxaccording to DIN EN 10020, where xx may be a natural number between 0and 99. Particularly preferred materials are the steels with materialsnumbers 1.4541 and 1.4571, especially steel with materials number1.4571.

Advantageously, the inner wall of the horizontal mixer which is incontact with the product is polished. Polished stainless steel surfaceshave a lower roughness and a lower contact angle with respect to waterthan matt or roughened steel surfaces.

The residence time in the horizontal mixer is preferably from 1 to 180minutes, more preferably from 2 to 60 minutes, most preferably from 5 to20 minutes.

The peripheral speed of the mixing tools in the horizontal mixer ispreferably from 0.1 to 10 m/s, more preferably from 0.5 to 5 m/s, mostpreferably from 0.75 to 2.5 m/s.

The surface postcrosslinked water-absorbing polymer particles are movedin the horizontal mixer at a speed which corresponds to a Froude numberof preferably 0.01 to 6, more preferably 0.05 to 3, most preferably 0.1to 0.7.

For mixers with horizontally mounted mixing tools, the Froude number isdefined as follows:

${Fr} = \frac{\omega^{2}r}{g}$where

-   -   r: radius of the mixing tool    -   ω: angular frequency    -   g: acceleration due to gravity.

The fill level of the horizontal mixer is preferably from 30 to 80%,more preferably from 40 to 75%, most preferably from 50 to 70%.

The aqueous liquid is preferably sprayed on by means of a two-substancenozzle, more preferably by means of an internally mixing two-substancenozzle.

Two-substance nozzles enable atomization into fine droplets or a spraymist. The atomization form employed is a circular or else ellipticalsolid or hollow cone. Two-substance nozzles may be configured withexternal mixing or internal mixing. In the case of the externally mixingtwo-substance nozzles, liquid and atomizer gas leave the nozzle headthrough separate orifices. They are mixed in the spray jet only afterleaving the spray nozzle. This enables independent regulation of dropletsize distribution and throughput over a wide range. The spray cone ofthe spray nozzle can be adjusted via the air cap setting. In the case ofthe internally mixing two-substance nozzle, liquid and atomizer gas aremixed within the spray nozzle and the biphasic mixture leaves the nozzlehead through the same bore (or through a plurality of parallel bores).In the case of the internally mixing two-substance nozzle, thequantitative ratios and pressure conditions are more highly coupled thanin the case of the externally mixing spray nozzle. Small changes in thethroughput therefore lead to a change in the droplet size distribution.The adjustment to the desired throughput is effected through theselected cross section of the nozzle bore.

Useful atomizer gases include compressed air, gas or steam of 0.5 barand more. The droplet size can be adjusted individually via the ratio ofliquid to atomizer gas, and also gas and liquid pressure.

In a particularly preferred embodiment, the liquid in the horizontalmixer is sprayed below the product bed surface of the moving polymerparticle layer, preferably at least 10 mm, more preferably at least 50mm, most preferably at least 100 mm, i.e. the spray nozzle is immersedinto the product bed.

The product bed surface is the interface which is established betweenthe surface postcrosslinked water-absorbing polymer particles which aremoved within the horizontal mixer and the blanketing atmosphere.

In the horizontal mixer, the angle between the mixer axis and the feedto the spray nozzle is preferably approx. 90°. The liquid can besupplied vertically from above. A feed obliquely from the side islikewise possible, in which case the angle relative to the vertical ispreferably between 60 and 90°, more preferably between 70 and 85°, mostpreferably between 75 and 82.5°. The oblique arrangement of the feedenables the use of shorter feeds and hence lower mechanical stressesduring the operation of the horizontal mixer.

In a particularly preferred embodiment, the spray nozzle in thehorizontal mixer is below the axis of rotation and sprays in thedirection of rotation. By virtue of this arrangement, the remoisturizedwater-absorbing polymer particles are conveyed optimally away from thespray nozzle. In combination with the oblique arrangement, it is alsopossible to exchange the spray nozzle during the operation of the mixer,without product escaping.

In a further preferred embodiment of the present invention, at least onespray nozzle is thermally insulated and/or trace-heated.

“Thermally insulated” means that the outer surface of the spray nozzleat least partly has a further material layer, the material of saidfurther material layer having a lower thermal conductivity than thematerial of the spray nozzle. The thermal conductivity of the materialof the further material layer at 20° C. is preferably less than 2μm⁻¹K⁻¹, more preferably less than 0.5 μm⁻¹K⁻¹, most preferably lessthan 0.1 μm⁻¹K⁻¹.

“Trace-heated” means that thermal energy is additionally supplied to thespray nozzle, for example by means of electrical energy or by means of aheating jacket through which a heat carrier flows. Suitable heatcarriers are commercial heat carrier oils, such as Marlotherm®, steam orhot water.

A possible supply of heat via one of the feedstocks used in the mixing,i.e. surface postcrosslinked water-absorbing polymer particles or liquidto be sprayed, is not trace heating in the context of the presentinvention.

The temperature of the spray nozzle is preferably from 1 to 20° C., morepreferably from 2 to 15° C., most preferably from 5 to 10° C., higherthan the temperature of the surface postcrosslinked water-absorbingpolymer particles.

In the case of a thermally insulated spray nozzle, the temperature ofthe liquid to be sprayed is preferably from 1 to 20° C., more preferablyfrom 2 to 15° C., most preferably from 5 to 10° C., higher than thetemperature of the surface postcrosslinked water-absorbing polymerparticles. The temperature of the liquid to be sprayed correspondsapproximately to the temperature of the spray nozzle.

In the case of a trace-heated and optionally thermally insulated spraynozzle, the temperature difference between the surface postcrosslinkedwater-absorbing polymer particles and the liquid to be sprayed on ispreferably less than 20° C., preferentially less than 10° C., morepreferably less than 5° C., most preferably less than 2° C.

The temperature difference between the liquid to be sprayed on and theatomizer gas is preferably less than 20° C., preferentially less than10° C., more preferably less than 5° C., most preferably less than 2° C.

The aqueous liquids usable for remoisturization i) preferably compriseinorganic particulate substances, inorganic colloidally dissolvedsubstances, organic polymers, cationic polymers and/or polyvalent metalcations. Suitable cationic polymers and/or polyvalent metal cations are,in a preferred embodiment, present before the remoisturization i) in theform of water-soluble salts thereof with organic or inorganic anions,and are used in the form of aqueous solutions or aqueous dispersions.

Suitable inorganic particulate substances (inorganic particles) are clayminerals such as montmorillonite, kaolinite and talc, water-insolublesulfates such as strontium sulfate, calcium sulfate and barium sulfate,carbonates such as magnesium carbonate, potassium carbonate and calciumcarbonate, salts of polyvalent metal cations such as aluminum sulfate,aluminum nitrate, aluminum chloride, potassium aluminum sulfate(potassium alum) and sodium aluminum sulfate (sodium alum), magnesiumsulfate, magnesium citrate, magnesium lactate, zirconium sulfate,zirconium lactate, iron lactate, iron citrate, calcium acetate, calciumpropionate, calcium citrate, calcium lactate, strontium lactate, zinclactate, zinc sulfate, zinc citrate, aluminum lactate, aluminum acetate,aluminum formate, calcium formate, strontium formate, strontium acetate,oxides such as magnesium oxide, aluminum oxide, zinc oxide, iron(II)oxide, zirconium dioxide and titanium dioxide, water-insolublephosphates such as magnesium phosphate, strontium phosphate, aluminumphosphate, iron phosphate, zirconium phosphate and calcium phosphate,diatomaceous earth, polysilicic acids, zeolites and activated carbons.Preference is given to using polysilicic acids which, according to themethod of preparation, are distinguished between precipitated silicasand fumed silicas. Both variants are commercially available under thenames Silica FK, Sipernat®, Wessalon® (precipitated silicas), orAerosil® (fumed silicas). Also advantageous are colloidal silicasolutions, in which the silica particles typically have a diameter ofless than 1 μm. Such solutions are available under the name Levasil®.

However, preference is given to using water-insoluble inorganicparticles, for example fumed silica, precipitated silica andwater-insoluble metal phosphates. Suitable water-insoluble inorganicparticles are described in DE 102 39 074 A1, and suitablewater-insoluble metal phosphates in U.S. Pat. No. 6,831,122, both ofwhich explicitly form part of the present disclosure.

In this context “water-insoluble” means a solubility in water at 23° C.of less than 1 g/100 g of water, preferably of less than 0.5 g/100 g ofwater, more preferably of less than 0.1 g/100 g of water, mostpreferably of less than 0.05 g/100 g of water.

Preferably fumed silicas and/or precipitated silicas are used asinorganic particles.

The inorganic particles have a mean particle size of preferably lessthan 400 μm, more preferably less than 100 μm, most preferably less than50 μm.

When inorganic particles are used, the amount used, based on thewater-absorbing polymer particles, is preferably from 0.05 to 5% byweight, more preferably from 0.1 to 1.5% by weight, most preferably from0.3 to 1% by weight.

Suitable organic polymers are all polyfunctional amines with primary orsecondary amino groups, such as polyethyleneimine, polyallylamine andpolylysine. The organic polymers preferred in the process according tothe invention are polyamines, such as polyvinylamine. Particularlysuitable organic polymers are N-containing polymers described in DE 10239 074 A1, which explicitly form part of the present disclosure. In apreferred embodiment, the partly hydrolyzed poly-N-vinylcarboxamidesdescribed there are used.

When organic polymers are used, the amount of organic polymer used,based on the water-absorbing polymer particles, is preferably from 0.1to 15% by weight, more preferably from 0.5 to 10% by weight, mostpreferably from 1 to 5% by weight.

Suitable cationic polymers are cationic derivatives of polyacrylamidesand polyquaternary amines. The anions of the cationic polymers used areall known organic and inorganic anions, preference being given tochloride, formate, acetate, propionate, malate, tartrate, citrate andlactate. The cationic polymers may, however, also be used, for example,in the form of sulfates, phosphates or carbonates, in which casesparingly water-soluble salts may form, which can be used in the form ofpowders or aqueous dispersions.

Polyquaternary amines are, for example, condensation products ofhexamethylenediamine, dimethylamine and epichlorohydrin, condensationproducts of dimethylamine and epichlorohydrin, copolymers ofhydroxyethylcellulose and diallyldimethylammonium chloride, copolymersof acrylamide and α-methacryloyloxyethyltrimethylammonium chloride,condensation products of hydroxyethylcellulose, epichlorohydrin andtrimethylamine, homopolymers of diallyldimethylammonium chloride, andaddition products of epichlorohydrin onto amidoamines. In addition, itis possible to obtain polyquaternary amines by reacting dimethyl sulfatewith polymers such as polyethyleneimines, copolymers of vinylpyrrolidoneand dimethylaminoethyl methacrylate or copolymers of ethyl methacrylateand diethylaminoethyl methacrylate. The polyquaternary amines areavailable in a wide molecular weight range.

When cationic polymers are used, the amount of cationic polymer used,based on the water-absorbing polymer particles, is preferably from 0.1to 15% by weight, more preferably from 0.5 to 10% by weight, mostpreferably from 1 to 5% by weight.

Suitable polyvalent metal cations are, for example, divalent cationssuch as the cations of zinc, magnesium, calcium, iron and strontium,trivalent cations such as the cations of aluminum, iron, chromium, rareearths and manganese, tetravalent cations such as the cations oftitanium and zirconium. Possible counterions are chloride, bromide,sulfate, hydrogensulfate, carbonate, hydrogencarbonate, nitrate,phosphate, hydrogenphosphate, dihydrogenphosphate and carboxylate, suchas acetate, citrate, tartrate and lactate. Aluminum sulfate, aluminumlactate and zirconium sulfate are preferred. In the presence ofdifferent diastereomers, as in the case of tartaric acid, all forms areincluded and can be used as anions for all polyvalent metal cationsusable in accordance with the invention. The polyvalent metal cationsare preferably used in the form of a solution.

Further particularly suitable polyvalent metal cations are described inWO 2005/080479 A1, which explicitly forms part of the presentdisclosure. It is also possible to use any desired mixtures of thesoluble salts of mono- and polyvalent metal cations; for example, it ispossible to prepare and use a suitable aqueous solution by dissolvinglactic acid or alkali metal lactate together with aluminum sulfate. Theprinciple can be generalized to any desired salts of polyvalent metalcations. It is also possible to use mixtures of different polyvalentmetal cations or any desired mixtures of salts thereof, for examplezirconium lactate and aluminum lactate, aluminum lactate and calciumlactate, zirconium lactate and calcium citrate.

Furthermore, it is possible for any desired organic and inorganic saltsof monovalent cations, preferably alkali metal salts, organic acidsand/or inorganic acids, additionally to be present in the solution withthe polyvalent metal cations. Examples thereof are alkali metalphosphates, alkali metal sulfates, alkali metal hydrogensulfates, alkalimetal dihydrogenphosphates, alkali metal hydrogencarbonates, alkalimetal hydrogensulfites, and formates, acetates, lactates, propionates,tartrates, citrates, malates of the alkali metals, of ammonium and oftriethanolammonium.

When polyvalent metal cations are used, the amount of polyvalent metalcation used, based on the water-absorbing polymer particles, istypically at least 0.0001% by weight, preferably from 0.005 to 5% byweight, more preferably from 0.05 to 1.5% by weight, most preferablyfrom 0.1 to 1% by weight.

The remoisturized polymer particles can subsequently be pneumaticallydelivered.

In principle, a distinction can be drawn between three differentdelivery types in pneumatic delivery.

-   -   In the case of aerial delivery and stream delivery in the region        of high gas velocities, the laws of the free-flowing individual        particle apply approximately. This is the classical type of        pneumatic delivery. No product deposits whatsoever occur. There        is essentially uniform delivery material distribution in the        tube.    -   When the gas velocity falls, the delivery moves into the range        of strand delivery, where the delivery material flows in the        lower half of the tube in particular. In the upper half of the        tube, there is aerial delivery.    -   At low gas velocities, the delivery proceeds extremely gently as        dense stream delivery (plug delivery, momentum delivery) with        high pressure drop.

In principle, pressure delivery can work with slower delivery rates thansuction delivery, since the pressure reserves under elevated pressureare greater than under reduced pressure, and since the delivery gasdensity which drives the product onward increases with rising pressure.

Since delivery gas is compressible, there is no constant pressure in thedelivery line, but rather a higher pressure at the start than at theend. However, this also changes the gas volume, so that, at the start,at higher pressure, slower gas velocities predominate, and, at the end,at lower pressure, higher gas velocities predominate.

H. Kalman, Powder Technology 104 (1999) 214 to 220 describesinvestigations of the attrition in pneumatic delivery systems. Owing tothe relatively low mechanical stress, relatively low delivery rates areadvantageous. According to the publication, often unnecessarily highdelivery rates are, however, often selected for safety reasons inpneumatic delivery.

Excessively low delivery rates in the region of strand delivery areproblematic, since stable delivery is not possible in the unstableregion between dense stream delivery and strand delivery. Instead, themechanical stresses which occur can lead to severe damage to thedelivery system, up to and including pulling of the delivery lines outof the mounts.

The optimal initial gas velocity in the pneumatic delivery ii) dependsupon the diameter of the delivery line. This dependence is bestdescribed with the Froude number. For pneumatic delivery, the Froudenumber is defined as follows:

${Fr} = \frac{v}{\sqrt{D \times g}}$v gas velocityD inner diameter of the transport lineg acceleration due to gravity.

The Froude number in the pneumatic delivery ii) of water-absorbingpolymer particles is preferably from 10 to 18, more preferably from 11to 16, most preferably from 12 to 14.

At excessively low delivery rates, the pneumatic delivery ii) becomesunstable, and relatively high delivery rates increase the undesiredattrition owing to rising mechanical stress.

The delivery material loading of the pneumatic delivery ii) ispreferably from 0.5 to 20 kg/kg, more preferably from 1 to 10 kg/kg,most preferably from 2 to 6 kg/kg, the delivery material loading beingthe quotient of delivery material mass flow rate and gas mass flow rate.

In principle, the optimal initial gas velocity also increases withrising delivery material loading.

The diameter of the pipeline in which the pneumatic delivery ii) iscarried out is preferably from 3 to 30 cm, more preferably from 4 to 25cm, most preferably from 5 to 20 cm. Excessively low tube diameters leadto a higher mechanical stress as a result of the pneumatic delivery ii)and hence promote the undesired attrition. Excessively large tubediameters enable an equally undesired settling of the water-absorbingpolymer particles in the delivery line.

In order to minimize mechanical stress, the number of curves in thepipeline of a pneumatic delivery system should be at a minimum,preferably fewer than 6, preferentially fewer than 5, more preferablyfewer than 4, most preferably fewer than 3. A pipeline in a pneumaticdelivery system is the section between the introduction unit for thewater-absorbing polymer particles and the receiving vessel, i.e. theregion in which the water-absorbing polymer particles are transported inthe gas stream.

The time (delay time) between the remoisturization i) and the pneumaticdelivery ii) is preferably less than 60 minutes, more preferably lessthan 45 minutes, most preferably less than 30 minutes.

The present invention is based on the finding that the water used in theremoisturization increases the elasticity of the water-absorbing polymerparticles. For the elasticity of the water-absorbing polymer particles,however, only the water close to the particle surface appears to beimportant. At the particle surface, the water concentration, however, isat its greatest shortly after the remoisturization. With time, the waterdiffuses slowly from the particle surface into the particle interior andthe water concentration at the particle surface falls again.

In the course of pneumatic delivery, the water-absorbing polymerparticles are mechanically stressed. Elastic water-absorbing polymerparticles are damaged less significantly. It is thus advantageous topneumatically deliver water-absorbing polymer particles immediatelyafter the remoisturization.

The remoisturized polymer particles are classified, which removesexcessively small and/or excessively large polymer particles, and theyare recycled into the process.

The screening machines suitable for the classification iii) are notsubject to any restriction. Preference is given to using tumblerscreening machines. Suitable tumbler screening machines are obtainable,for example, from ALLGAIER Werke GmbH, Uhingen, Germany, and MINOXSiebtechnik GmbH, Offenbach/Queich, Germany.

In a tumbler screening machine, the water-absorbing polymer particles tobe classified are moved over the screen in a spiral manner owing to aforced vibration. The relatively long screening distance coupled with asmall screening area leads to a high sharpness of separation in theclassification. The forced vibration typically has an amplitude of 0.7to 40 mm, preferably of 1.5 to 25 mm, and a frequency of 1 to 100 Hz,preferably of 5 to 10 Hz.

The tumbler screening machines preferably have at least 2, morepreferably at least 3 and most preferably at least 4 screens.Advantageously, the water-absorbing polymer particles falling down fromthe upper screen are deflected by a preferably funnel-shaped apparatusin the direction of the middle of the lower screen.

The mesh size of the screens is preferably in the range from 100 to 1000μm, more preferably in the range from 150 to 850 μm, most preferably inthe range from 150 to 600 μm.

The water-absorbing polymer particles preferably have a temperatureduring the classification of 40 to 120° C., more preferably of 45 to100° C., most preferably of 50 to 80° C.

The classification is particularly advantageously performedcontinuously. The throughput of water-absorbing polymer particles istypically at least 100 kg/m²·h, preferably at least 150 kg/m²·h,preferentially at least 200 kg/m²·h, more preferably at least 250kg/m²·h, most preferably at least 300 kg/m²·h.

A gas stream, more preferably air, preferably flows over thewater-absorbing polymer particles during the classification. The gasrate is typically from 0.1 to 10 m³/h per m² of screen area, preferablyfrom 0.5 to 5 m³/h per m² of screen area, more preferably from 1 to 3m³/h per m² of screen area, the gas volume being measured under standardconditions (25° C. and 1 bar). The gas stream is more preferably heatedslightly before entry into the screening apparatus, typically to atemperature of 40 to 120° C., preferably to a temperature of 50 to 110°C., preferentially to a temperature of 60 to 100° C., more preferably toa temperature of 65 to 90° C., most preferably to a temperature of 70 to80° C. The water content of the gas stream is typically less than 5g/kg, preferably less than 4.5 g/kg, preferentially less than 4 g/kg,more preferably less than 3.5 g/kg, most preferably less than 3 g/kg. Agas stream with a low water content can be obtained, for example, bycondensing an appropriate amount of water out of a gas stream withhigher water content by cooling.

In a preferred embodiment of the present invention, a plurality oftumbler screening machines are operated in parallel.

The tumbler screening machines are typically electrically grounded.

The time (delay time) between the remoisturization i) and theclassification iii) is preferably at least 30 minutes, more preferablyat least 45 minutes, most preferably at least 60 minutes.

The present invention is based on the finding that the water used in theremoisturization lowers the glass transition temperature of thewater-absorbing polymer particles; the particle surface becomes tacky.At the particle surface, the water concentration, however, is at itsgreatest shortly after the remoisturization. With time, the waterdiffuses slowly from the particle surface into the particle interior andthe water concentration at the particle surface falls again. Thetackiness of the particle surface thus passes through a maximum.

However, the tackiness of the particle surface causes very smallparticles to adhere firmly to larger particles, as a result of whichthese adhering, very small particles can be removed by classificationonly with difficulty. It is therefore advantageous, after theremoisturization, still to delay the classification for a sufficienttime. Within this time, some of the water diffuses into the particlesurface and the tackiness of the particle surface decreases again.

In order to achieve the desired delay time between remoisturization i)and classification iii), the water-absorbing polymer particles can bestored intermediately in suitable vessels.

The production of the surface postcrosslinked polymer particles isexplained in detail hereinafter:

The water-absorbing polymer particles are produced by polymerizing amonomer solution or suspension and are typically water-insoluble.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 35 g/100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid and itaconicacid. Particularly preferred monomers are acrylic acid and methacrylicacid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturatedsulfonic acids, such as styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities can have a considerable influence on the polymerization. Theraw materials used should therefore have a maximum purity. It istherefore often advantageous to specially purify the monomers a).Suitable purification processes are described, for example, in WO2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitablemonomer a) is, for example, acrylic acid purified according to WO2004/035514 A1 comprising 99.8460% by weight of acrylic acid, 0.0950% byweight of acetic acid, 0.0332% by weight of water, 0.0203% by weight ofpropionic acid, 0.0001% by weight of furfurals, 0.0001% by weight ofmaleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% byweight of hydroquinone monomethyl ether.

The proportion of acrylic acid and/or salts thereof in the total amountof monomers a) is preferably at least 50 mol %, more preferably at least90 mol %, most preferably at least 95 mol %.

The monomers a) typically comprise polymerization inhibitors, preferablyhydroquinone monoethers, as storage stabilizers.

The monomer solution comprises preferably up to 250 ppm by weight,preferably at most 130 ppm by weight, more preferably at most 70 ppm byweight, preferably at least 10 ppm by weight, more preferably at least30 ppm by weight, especially around 50 ppm by weight, of hydroquinonemonoether, based in each case on the unneutralized monomer a). Forexample, the monomer solution can be prepared by using an ethylenicallyunsaturated monomer bearing acid groups with an appropriate content ofhydroquinone monoether.

Preferred hydroquinone monoethers are hydroquinone monomethyl ether(MEHQ) and/or alpha-tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groupssuitable for crosslinking. Such groups are, for example, ethylenicallyunsaturated groups which can be polymerized free-radically into thepolymer chain, and functional groups which can form covalent bonds withthe acid groups of the monomer a). In addition, polyvalent metal saltswhich can form coordinate bonds with at least two acid groups of themonomer a) are also suitable as crosslinkers b).

Crosslinkers b) are preferably compounds having at least twopolymerizable groups which can be polymerized free-radically into thepolymer network. Suitable crosslinkers b) are, for example, ethyleneglycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycoldiacrylate, allyl methacrylate, trimethylolpropane triacrylate,triallylamine, tetraallylammonium chloride, tetraallyloxyethane, asdescribed in EP 0 530 438 A1, di- and triacrylates, as described in EP 0547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 103 31 450A1, mixed acrylates which, as well as acrylate groups, comprise furtherethylenically unsaturated groups, as described in DE 103 31 456 A1 andDE 103 55 401 A1, or crosslinker mixtures, as described, for example, inDE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/032962A2.

Preferred crosslinkers b) are pentaerythrityl triallyl ether,tetraalloxyethane, methylenebismethacrylamide, 15-tuply ethoxylatedtrimethylolpropane triacrylate, polyethylene glycol diacrylate,trimethylolpropane triacrylate and triallylamine.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample, in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol, especially thetriacrylate of 3-tuply ethoxylated glycerol.

The amount of crosslinker b) is preferably 0.05 to 1.5% by weight, morepreferably 0.1 to 1% by weight, most preferably 0.3 to 0.6% by weight,based in each case on monomer a). With rising crosslinker content, thecentrifuge retention capacity (CRC) falls and the absorption under apressure of 21.0 g/cm² passes through a maximum.

The initiators c) used may be all compounds which generate free radicalsunder the polymerization conditions, for example thermal initiators,redox initiators, photoinitiators. Suitable redox initiators are sodiumperoxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid, sodiumperoxodisulfate/sodium bisulfite and hydrogen peroxide/sodium bisulfite.Preference is given to using mixtures of thermal initiators and redoxinitiators, such as sodium peroxodisulfate/hydrogen peroxide/ascorbicacid. The reducing component used is, however, preferably a mixture ofthe sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium saltof 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixturesare obtainable as Brüggolite® FF6 and Brüggolite® FF7 (BrüggemannChemicals; Heilbronn; Germany).

Ethylenically unsaturated monomers d) copolymerizable with theethylenically unsaturated monomers a) bearing acid groups are, forexample, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethylmethacrylate, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.

The water-soluble polymers e) used may be polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, modified cellulose,such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycolsor polyacrylic acids, preferably starch, starch derivatives and modifiedcellulose.

Typically, an aqueous monomer solution is used. The water content of themonomer solution is preferably from 40 to 75% by weight, more preferablyfrom 45 to 70% by weight, most preferably from 50 to 65% by weight. Itis also possible to use monomer suspensions, i.e. monomer solutions withexcess monomer a), for example sodium acrylate. With rising watercontent, the energy requirement in the subsequent drying rises, and,with falling water content, the heat of polymerization can only beremoved inadequately.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. The monomer solution can therefore be freed ofdissolved oxygen, and the polymerization inhibitor present in themonomer solution can be deactivated, before the polymerization byinertization, i.e. flowing an inert gas through, preferably nitrogen orcarbon dioxide. The oxygen content of the monomer solution is preferablylowered before the polymerization to less than 1 ppm by weight, morepreferably to less than 0.5 ppm by weight, most preferably to less than0.1 ppm by weight.

Suitable reactors are, for example, kneading reactors or belt reactors.In the kneader, the polymer gel formed in the polymerization of anaqueous monomer solution or suspension is comminuted continuously by,for example, contrarotatory stirrer shafts, as described in WO2001/038402 A1. Polymerization on a belt is described, for example, inDE 38 25 366 A1 and U.S. Pat. No. 6,241,928. Polymerization in a beltreactor forms a polymer gel, which has to be comminuted in a furtherprocess step, for example in an extruder or kneader.

To improve the drying properties, the comminuted polymer gel obtained bymeans of a kneader can additionally be extruded.

However, it is also possible to dropletize an aqueous monomer solutionand to polymerize the droplets obtained in a heated carrier gas stream.This allows the process steps of polymerization and drying to becombined, as described in WO 2008/040715 A2 and WO 2008/052971 A1.

The acid groups of the resulting polymer gels have typically beenpartially neutralized. Neutralization is preferably carried out at themonomer stage. This is typically done by mixing in the neutralizingagent as an aqueous solution or preferably also as a solid. The degreeof neutralization is preferably from 25 to 95 mol %, more preferablyfrom 30 to 80 mol %, most preferably from 40 to 75 mol %, for which thecustomary neutralizing agents can be used, preferably alkali metalhydroxides, alkali metal oxides, alkali metal carbonates or alkali metalhydrogencarbonates and also mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonium salts. Particularly preferredalkali metals are sodium and potassium, but very particular preferenceis given to sodium hydroxide, sodium carbonate or sodiumhydrogencarbonate and also mixtures thereof.

However, it is also possible to carry out neutralization after thepolymerization, at the stage of the polymer gel formed in thepolymerization. It is also possible to neutralize up to 40 mol %,preferably 10 to 30 mol % and more preferably 15 to 25 mol % of the acidgroups before the polymerization by adding a portion of the neutralizingagent actually to the monomer solution and setting the desired finaldegree of neutralization only after the polymerization, at the polymergel stage. When the polymer gel is neutralized at least partly after thepolymerization, the polymer gel is preferably comminuted mechanically,for example by means of an extruder, in which case the neutralizingagent can be sprayed, sprinkled or poured on and then carefully mixedin. To this end, the gel mass obtained can be repeatedly extruded forhomogenization.

The polymer gel is then preferably dried with a belt drier until theresidual moisture content is preferably 0.5 to 15% by weight, morepreferably 1 to 10% by weight, most preferably 2 to 8% by weight, theresidual moisture content being determined by EDANA recommended testmethod No. WSP 230.2-05 “Moisture Content”. In the case of too high aresidual moisture content, the dried polymer gel has too low a glasstransition temperature T_(g) and can be processed further only withdifficulty. In the case of too low a residual moisture content, thedried polymer gel is too brittle and, in the subsequent comminutionsteps, undesirably large amounts of polymer particles with anexcessively low particle size (fines) are obtained. The solids contentof the gel before the drying is preferably from 25 to 90% by weight,more preferably from 35 to 70% by weight, most preferably from 40 to 60%by weight. Optionally, it is, however, also possible to use a fluidizedbed drier or a paddle drier for the drying operation.

Thereafter, the dried polymer gel is ground and classified, and theapparatus used for grinding may typically be single- or multistage rollmills, preferably two- or three-stage roll mills, pin mills, hammermills or vibratory mills.

The mean particle size of the polymer particles removed as the productfraction is preferably at least 200 μm, more preferably from 250 to 600μm, very particularly from 300 to 500 μm. The mean particle size of theproduct fraction may be determined by means of EDANA recommended testmethod No. WSP 220.2-05 “Particle Size Distribution”, where theproportions by mass of the screen fractions are plotted in cumulativeform and the mean particle size is determined graphically. The meanparticle size here is the value of the mesh size which gives rise to acumulative 50% by weight.

The proportion of particles with a particle size of at least 150 μm ispreferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

Polymer particles with too small a particle size lower the permeability(SFC). The proportion of excessively small polymer particles (fines)should therefore be small.

Excessively small polymer particles are therefore typically removed andrecycled into the process. This is preferably done before, during orimmediately after the polymerization, i.e. before the drying of thepolymer gel. The excessively small polymer particles can be moistenedwith water and/or aqueous surfactant before or during the recycling.

It is also possible in later process steps to remove excessively smallpolymer particles, for example after the surface postcrosslinking oranother coating step. In this case, the excessively small polymerparticles recycled are surface postcrosslinked or coated in another way,for example with fumed silica.

When a kneading reactor is used for polymerization, the excessivelysmall polymer particles are preferably added during the last third ofthe polymerization.

When the excessively small polymer particles are added at a very earlystage, for example actually to the monomer solution, this lowers thecentrifuge retention capacity (CRC) of the resulting water-absorbingpolymer particles. However, this can be compensated, for example, byadjusting the amount of crosslinker b) used.

When the excessively small polymer particles are added at a very latestage, for example not until an apparatus connected downstream of thepolymerization reactor, for example to an extruder, the excessivelysmall polymer particles can be incorporated into the resulting polymergel only with difficulty. Insufficiently incorporated, excessively smallpolymer particles are, however, detached again from the dried polymergel during the grinding, are therefore removed again in the course ofclassification and increase the amount of excessively small polymerparticles to be recycled.

The proportion of particles having a particle size of at most 850 μm ispreferably at least 90% by weight, more preferably at least 95% byweight, most preferably at least 98% by weight.

Advantageously, the proportion of particles having a particle size of atmost 600 μm is preferably at least 90% by weight, more preferably atleast 95% by weight, most preferably at least 98% by weight.

Polymer particles with too great a particle size lower the swell rate.The proportion of excessively large polymer particles should thereforelikewise be small.

Excessively large polymer particles are therefore typically removed andrecycled into the grinding of the dried polymer gel.

To further improve the properties, the polymer particles are surfacepostcrosslinked. Suitable surface postcrosslinkers are compounds whichcomprise groups which can form covalent bonds with at least twocarboxylate groups of the polymer particles. Suitable compounds are, forexample, polyfunctional amines, polyfunctional amidoamines,polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described inDE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, orβ-hydroxyalkylamides, as described in DE 102 04 938 A1 and U.S. Pat. No.6,239,230.

Additionally described as suitable surface postcrosslinkers are cycliccarbonates in DE 40 20 780 C1, 2-oxazolidone and its derivatives, suchas 2-hydroxyethyl-2-oxazolidone in DE 198 07 502 A1, bis- andpoly-2-oxazolidinones in DE 198 07 992 C1, 2-oxotetrahydro-1,3-oxazineand its derivatives in DE 198 54 573 A1, N-acyl-2-oxazolidones in DE 19854 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amide acetals inDE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327 A2 andmorpholine-2,3-dione and its derivatives in WO 2003/031482 A1.

Preferred surface postcrosslinkers are glycerol, ethylene carbonate,ethylene glycol diglycidyl ether, reaction products of polyamides withepichlorohydrin, and mixtures of propylene glycol and 1,4-butanediol.

Very particularly preferred surface postcrosslinkers are2-hydroxyethyloxazolidin-2-one, oxazolidin-2-one and 1,3-propanediol.

In addition, it is also possible to use surface postcrosslinkers whichcomprise additional polymerizable ethylenically unsaturated groups, asdescribed in DE 37 13 601 A1.

The amount of surface postcrosslinkers is preferably 0.001 to 2% byweight, more preferably 0.02 to 1% by weight, most preferably 0.05 to0.2% by weight, based in each case on the polymer particles.

In a preferred embodiment of the present invention, polyvalent cationsare applied to the particle surface in addition to the surfacepostcrosslinkers before, during or after the surface postcrosslinking.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium, iron and strontium, trivalent cations such as thecations of aluminum, iron, chromium, rare earths and manganese,tetravalent cations such as the cations of titanium and zirconium.Possible counterions are chloride, bromide, sulfate, hydrogensulfate,carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphosphate,dihydrogenphosphate and carboxylate, such as acetate, citrate andlactate. Aluminum sulfate and aluminum lactate are preferred. Apart frommetal salts, it is also possible to use polyfunctional amines aspolyvalent cations. Polyfunctional amines are compounds with at leasttwo amino and/or ammonium groups. Preference is given, however, to usingmetal cations as polyvalent cations.

The amount of polyvalent cation used is, for example, 0.001 to 1.5% byweight, preferably 0.005 to 1% by weight, more preferably 0.02 to 0.8%by weight, based in each case on the polymer particles.

The surface postcrosslinking is typically performed in such a way that asolution of the surface postcrosslinker is sprayed onto the driedpolymer particles. After the spraying, the polymer particles coated withsurface postcrosslinker are dried thermally, and the surfacepostcrosslinking reaction can take place either before or during thedrying.

The spray application of a solution of the surface postcrosslinker ispreferably performed in mixers with moving mixing tools, such as screwmixers, disk mixers and paddle mixers. Particular preference is given tohorizontal mixers such as paddle mixers, very particular preference tovertical mixers. The distinction between horizontal mixers and verticalmixers is made by the position of the mixing shaft, i.e. horizontalmixers have a horizontally mounted mixing shaft and vertical mixers avertically mounted mixing shaft. Suitable mixers are, for example,horizontal Pflugschar® plowshare mixers (Gebr. Lödige Maschinenbau GmbH;Paderborn; Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BV;Doetinchem; the Netherlands), Processall Mixmill mixers (ProcessallIncorporated; Cincinnati; US) and Schugi Flexomix® (Hosokawa Micron BV;Doetinchem; the Netherlands). However, it is also possible to spray onthe surface postcrosslinker solution in a fluidized bed.

The surface postcrosslinkers are typically used in the form of anaqueous solution. The content of nonaqueous solvent and/or total amountof solvent can be used to adjust the penetration depth of the surfacepostcrosslinker into the polymer particles.

When exclusively water is used as the solvent, a surfactant isadvantageously added. This improves the wetting performance and reducesthe tendency to form lumps. However, preference is given to usingsolvent mixtures, for example isopropanol/water, 1,3-propanediol/waterand propylene glycol/water, where the mixing ratio by mass is preferablyfrom 20:80 to 40:60.

The thermal drying is preferably carried out in contact driers, morepreferably paddle driers, most preferably disk driers. Suitable driersare, for example, Hosokawa Bepex® horizontal paddle driers (HosokawaMicron GmbH; Leingarten; Germany), Hosokawa Bepex® disk driers (HosokawaMicron GmbH; Leingarten; Germany) and Nara paddle driers (NARA MachineryEurope; Frechen; Germany). Moreover, it is also possible to usefluidized bed driers.

The drying can be effected in the mixer itself, by heating the jacket orblowing in warm air. Equally suitable is a downstream drier, for examplea shelf drier, a rotary tube oven or a heatable screw. It isparticularly advantageous to mix and dry in a fluidized bed drier.

Preferred drying temperatures are in the range of 100 to 250° C.,preferably 120 to 220° C., more preferably 130 to 210° C., mostpreferably 150 to 200° C. The preferred residence time at thistemperature in the reaction mixer or drier is preferably at least 10minutes, more preferably at least 20 minutes, most preferably at least30 minutes, and typically at most 60 minutes.

The water-absorbing polymer particles produced by the process accordingto the invention have a moisture content of preferably 1 to 15% byweight, more preferably 1.5 to 10% by weight, most preferably 2 to 8% byweight, the water content being determined by EDANA recommended testmethod No. WSP 230.2-05 “Moisture Content”.

The water-absorbing polymer particles produced by the process accordingto the invention have a centrifuge retention capacity (CRC) of typicallyat least 15 g/g, preferably at least 20 g/g, preferentially at least 22g/g, more preferably at least 24 g/g, most preferably at least 26 g/g.The centrifuge retention capacity (CRC) of the water-absorbing polymerparticles is typically less than 60 g/g. The centrifuge retentioncapacity (CRC) is determined by EDANA recommended test method No. WSP241.2-05 “Centrifuge Retention Capacity”.

The water-absorbing polymer particles produced by the process accordingto the invention have an absorption under a pressure of 49.2 g/cm² oftypically at least 15 g/g, preferably at least 20 g/g, preferentially atleast 22 g/g, more preferably at least 24 g/g, most preferably at least26 g/g. The absorption under a pressure of 49.2 g/cm² of thewater-absorbing polymer particles is typically less than 35 g/g. Theabsorption under a pressure of 49.2 g/cm² is determined analogously toEDANA recommended test method No. WSP 242.2-05 “Absorption underPressure”, except that a pressure of 49.2 g/cm² is established insteadof a pressure of 21.0 g/cm².

The water-absorbing polymer particles are tested by means of the testmethods described below.

Methods:

The measurements should, unless stated otherwise, be carried out at anambient temperature of 23±2° C. and a relative air humidity of 50±10%.The water-absorbing polymer particles are mixed thoroughly before themeasurement.

Saline Flow Conductivity

The saline flow conductivity (SFC) of a swollen gel layer under apressure of 0.3 psi (2070 Pa) is, as described in EP 0 640 330 A1,determined as the gel layer permeability of a swollen gel layer ofwater-absorbing polymer particles, the apparatus described on page 19and in FIG. 8 in the aforementioned patent application having beenmodified to the effect that the glass frit (40) is not used, and theplunger (39) consists of the same polymer material as the cylinder (37)and now comprises 21 bores of equal size distributed homogeneously overthe entire contact area. The procedure and evaluation of the measurementremain unchanged from EP 0 640 330 A1. The flow is detectedautomatically.

The saline flow conductivity (SFC) is calculated as follows:SFC[cm³s/g]=(Fg(t=0)×L0)/(d×A×WP)where Fg(t=0) is the flow of NaCl solution in g/s, which is obtainedusing linear regression analysis of the Fg(t) data of the flowdeterminations by extrapolation to t=0, L0 is the thickness of the gellayer in cm, d is the density of the NaCl solution in g/cm³, A is thearea of the gel layer in cm², and WP is the hydrostatic pressure overthe gel layer in dyn/cm².Centrifuge Retention Capacity

The centrifuge retention capacity (CRC) is determined by EDANArecommended test method No. WSP 241.2-05 “Centrifuge RetentionCapacity”.

Absorption Under a Pressure of 49.2 g/cm²

The absorption under a pressure of 49.2 g/cm² (AUL0.7 psi) is determinedanalogously to EDANA (European Disposables and Nonwovens Association)recommended test method No. WSP 242.2-05 “Absorption under Pressure”,except that a pressure of 49.2 g/cm² (AUL0.7 psi) is established insteadof a pressure of 21.0 g/cm² (AUL0.3 psi).

The EDANA test methods are obtainable, for example, from EDANA, AvenueEugène Plasky 157, β-1030 Brussels, Belgium.

EXAMPLES Example 1 Production of the Water-Absorbing Polymer Particles

By continuously mixing deionized water, 50% by weight sodium hydroxidesolution and acrylic acid, an acrylic acid/sodium acrylate solution wasprepared, such that the degree of neutralization corresponds to 71.3 mol%. The solids content of the monomer solution was 38.8% by weight.

The polyethylenically unsaturated crosslinker used was polyethyleneglycol-400 diacrylate (diacrylate proceeding from a polyethylene glycolwith a mean molar mass of 400 g/mol). The amount used was 2 kg ofcrosslinker per t of monomer solution.

To initiate the free-radical polymerization, 1.03 kg of a 0.25% byweight aqueous hydrogen peroxide solution, 3.10 kg of a 15% by weightaqueous sodium peroxodisulfate solution and 1.05 kg of a 1% by weightaqueous ascorbic acid solution were used per t of monomer solution.

The throughput of the monomer solution was 20 t/h. The reaction solutionhad a temperature of 23.5° C. at the feed.

The individual components were metered in the following amountscontinuously into a List Contikneter continuous kneader reactor with acapacity of 6.3 m³ (LIST AG, Arisdorf, Switzerland):

20 t/h of monomer solution 40 kg/h of polyethylene glycol-400 diacrylate82.6 kg/h of hydrogen peroxide solution/sodium peroxodisulfate solution21 kg/h of ascorbic acid solution

Between the addition point for the crosslinker and the addition sitesfor the initiators, the monomer solution was inertized with nitrogen.

After approx. 50% of the residence time, a metered addition of fines(1000 kg/h), which were obtained from the production process by grindingand screening, to the reactor additionally took place. The residencetime of the reaction mixture in the reactor was 15 minutes.

The resulting polymer gel was placed onto a belt dryer. On the beltdryer, an air/gas mixture flowed continuously around the polymer gel anddried it. The residence time in the belt dryer was 37 minutes.

The dried polymer gel was ground and screened off to a particle sizefraction of 150 to 850 μm. The resulting base polymer was surfacepostcrosslinked.

In a Schugi Flexomix® (Hosokawa Micron B.V., Doetinchem, theNetherlands), the base polymer was coated with a surface postcrosslinkersolution and then dried in a NARA paddle dryer (GMF Gouda, Waddinxveen,the Netherlands) at 190° C. for 45 minutes.

The following amounts were metered into the Schugi Flexomix®:

7.5 t/h of base polymer 270.0 kg/h of surface postcrosslinker solution

The surface postcrosslinker solution comprised 2.8% by weight of2-hydroxyethyl-2 oxazolidone, 2.8% by weight of aluminum sulfate, 66.1%by weight of deionized water and 28.3% by weight of isopropanol.

After being dried, the surface postcrosslinked base polymer was cooledto approx. 60° C. in a NARA paddle cooler (GMF Gouda, Waddinxveen, theNetherlands).

The resulting water-absorbing polymer particles had a centrifugeretention capacity (CRC) of 28.4 g/g.

Example 2

A ProfiMixx 47 food processor (Robert Bosch GmbH;Gerlingen-Schillerhöhe; Germany) was initially charged with 200 g ofwater-absorbing polymer particles from example 1, which wereremoisturized with 5.0 g of water while stirring (level 4; approx. 500rpm) and stirred for a further minute. The water was sprayed on by meansof a peristaltic pump at a metering rate of 5 g/min. The tube had aninternal diameter of 2.54 mm.

With 20 g of the remoisturized water-absorbing polymer particles in eachcase, the particle size distribution was measured after 10, 20, 30, 40,50 and 60 minutes, and after 24 hours. The percentages reported arepercent by weight. The results are summarized in table 1:

TABLE 1 Remoisturization with 2.5% by weight of water <150 150-180180-300 >850 Minutes μm μm μm 300-600 μm 600-850 μm μm 10 0.3% 0.4% 8.6%48.2% 36.1% 6.3% 20 0.6% 0.4% 9.3% 48.7% 35.4% 5.5% 30 0.7% 0.5% 10.2%49.2% 34.2% 5.2% 40 0.8% 0.6% 10.2% 49.6% 34.2% 4.7% 50 0.9% 0.5% 9.9%47.0% 33.3% 8.4% 60 1.2% 0.8% 12.0% 50.6% 29.2% 6.2% 1440 1.5% 1.1%12.2% 50.5% 31.7% 3.2%

Example 3

The procedure was as in example 2. Instead of remoisturizing with 5.0 gof water, 10.0 g of water were used.

Subsequently, the particle size distribution was measured as a functionof time. The percentages reported are percent by weight. The results aresummarized in table 2:

TABLE 2 Remoisturization with 5% by weight of water <150 150-180 180-300300-600 Minutes μm μm μm μm 600-850 μm >850 μm 10 0.0% 0.0% 0.0% 0.7%1.1% 98.3% 20 0.2% 0.0% 0.9% 9.3% 25.8% 63.9% 30 0.1% 0.0% 1.7% 20.3%42.7% 35.2% 40 0.0% 0.2% 2.2% 23.4% 41.4% 33.0% 50 0.2% 0.1% 2.2% 22.6%40.7% 34.4% 60 0.1% 0.3% 2.5% 25.3% 40.2% 31.6% 1440 0.3% 0.3% 6.0%40.0% 45.0% 8.5%

In each of tables 1 and 2, a significant rise in small polymer particles(<150 μm) is discernible with increasing delay time betweenremoisturization and classification. Complete removal of excessivelysmall particles is thus possible only after a sufficient delay time. Theproportion of large particles (>850 μm), in contrast, decreases withtime.

The invention claimed is:
 1. A process for producing water-absorbingpolymer particles by polymerizing a monomer solution or suspensioncomprising a) at least one ethylenically unsaturated monomer which bearsa carboxylic acid group and may be at least partly neutralized, b) atleast one crosslinker, c) at least one initiator, d) optionally one ormore ethylenically unsaturated monomer copolymerizable with the monomermentioned under a), and e) optionally one or more water-solublepolymers, comprising drying, grinding, classifying, and surfacepostcrosslinking using a surface postcrosslinker to form covalent bondswith at least two carboxylate groups of the polymer particles andselected from the group consisting of glycerol, ethylene carbonate, areaction product of a polyamide with epichlorohydrin, a mixture ofpropylene glycol and 1,4-butanediol, a 2-hydroxyethyloxazolidin-2-one,oxazolidin-2-one, and 1,3-propanediol, in which i) the surfacepostcrosslinked polymer particles are remoisturized, ii) optionally, theremoisturized polymer particles are pneumatically delivered, iii) theremoisturized polymer particles are classified using a tumbler screeningmachine having at least two screens to remove polymer particles having aparticle size of less than 150 μm from the remainder of the polymerparticles, and iv) recycling the excessively small polymer particlesinto the process for producing water-absorbing polymer particles before,during, or immediately after the polymerization of the monomer solution,wherein a time between the remoisturization i) and the classificationiii) is at least 15 minutes.
 2. The process according to claim 1,wherein polyvalent cations have been used additionally in the surfacepostcrosslinking.
 3. The process according to claim 1, wherein thewater-absorbing polymer particles supplied to the remoisturization i)have a temperature of 40 to 80° C.
 4. The process according to claim 1,wherein remoisturization i) is effected using an aqueous solution or anaqueous dispersion comprising an inorganic particulate substance, aninorganic colloidally dissolved substance, an organic polymer, acationic polymer, and/or a salt of a polyvalent cation.
 5. The processaccording to claim 1, wherein the water-absorbing polymer particlessupplied to the classification iii) have a temperature of 40 to 80° C.6. The process according to claim 1, wherein at least 95% by weight ofthe water-absorbing polymer particles have a particle size of at least150 μm.
 7. The process according to claim 1, wherein at least 95% byweight of the water-absorbing polymer particles have a particle size ofat most 600 μm.
 8. The process according to claim 1, wherein thewater-absorbing polymer particles have a centrifuge retention capacityof at least 15 g/g.